Abstract
Purpose
Our aim was to determine if acetabular component position, femoral offset restoration, or leg-length equality is most important for total hip arthroplasty (THA) stability.
Methods
A matched case (n = 67)-control (n = 247) design and conditional logistic regression model were used to examine risk factors for dislocation in primary THA.
Results
When femoral offset was at least 3 mm greater than that of the contralateral hip, risk of dislocation was lower (p = 0.0192). Neither leg-length difference nor acetabular component abduction or version angle was associated with dislocation.
Conclusions
Our data suggest restoring femoral offset is the most important technical factor in preventing THA dislocation.
Keywords: Dislocation, Femoral offset, Total hip arthroplasty, Component position
1. Introduction
Total hip arthroplasty (THA) is widely performed and its frequency is expected to increase substantially over the next two decades.1, 2, 3, 4 One of the primary goals of THA is to provide a stable hip by restoring joint biomechanics. Failure to achieve this goal results in dislocation, and occurs regardless of surgical approach.5 THA dislocation requires closed reduction, adherence to hip precautions, and at times cumbersome bracing. Recurrent dislocation refractory to conservative management necessitates revision surgery, which has a significant risk of infection, hospital readmission, and in-hospital mortality.6, 7, 8 Thus, efforts to minimize the risk of THA dislocation are essential.
Proper component position and restoration of soft tissue tension are widely accepted technical goals during THA, but the extent to which each of these factors contributes to THA stability is unclear.9, 10, 11, 12, 13, 14, 15 Elucidating the relative impact of acetabular component position, femoral offset restoration, and leg-length equality on THA stability may help guide intraoperative decision-making to minimize the risk of postoperative dislocation.
The aims of this study were to determine: (1) if acetabular component position, femoral offset restoration, or leg-length equality is most important for THA stability; and (2) if patient characteristics or surgical factors predict THA dislocation.
2. Materials and methods
2.1. Study design
We examined the risk factors for THA dislocation using a retrospective matched case-control design. Institutional Review Board at our hospital approved this study. This retrospective study did not require formal consent.
The source population was all patients who underwent primary THA via the posterior approach by fellowship-trained adult reconstruction surgeons at our orthopaedic tertiary referral center from 2002 to 2012 (n = 17,329). Cases were defined as patients who required subsequent THA closed reduction performed at our institution (n = 67). Controls, THA patients who did not require closed reduction, were matched at a ratio of 4 controls per case by surgeon and year of surgery (n = 247).
2.2. Outcomes and statistical analysis
Risk factors of interest were acetabular component abduction angle and version, femoral offset difference, and leg-length difference. Each of these was measured on the first postoperative standing AP pelvis radiograph taken at our institution per standard protocol. Acetabular component version was measured with digital edge-detection method software (TraumaCad, Voyant Health, Petach-Tikva, Israel). This software calculates version by analyzing the elliptical radiographic profile of a hemispherical acetabular component relative to a line along the inferior aspect of the ischial tuberosities. Anteversion or retroversion was determined using a cross-table lateral radiograph. The remainder of the measurements were made digitally using a medical software package (AGFA IMPAX Orthopaedic Tools, Greenville, SC) integrated into our institution’s Picture Archiving and Communication System (PACS). Acetabular component abduction angle was determined by the angle between a line along the face of the acetabular component and a line along the inferior aspect of the ischial tuberosities. Femoral offset was determined by measuring the distance from the center of the femoral head to the tip of the greater trochanter along a line perpendicular to the long axis of the femur. Femoral offset difference was calculated by measuring femoral offset of the replaced hip and subtracting from it femoral offset of the contralateral hip. Leg-length was determined by extending a line along the inferior aspect of the ischial tuberosities and measuring the perpendicular distance to this line from the superior aspect of the lesser trochanter. To obtain leg-length difference, the above value of the contralateral hip was subtracted from that of the replaced hip.
The risk of dislocation was assessed utilizing a conditional logistic regression model, where outcome was case versus control status. The model was adjusted for age, gender, body mass index (BMI), and prosthetic head size. With type I error set at 5%, 67 matched sets of cases and controls (1 case matched to 4 controls), and an odds ratio of 2.6 for dislocation, we anticipated statistical power >0.80 to detect this difference.
3. Results
After matching controls to cases, there were no differences between groups with regard to operating surgeon or year of surgery (Table 1). The distribution of case and control patients across the adjusted covariates of age, BMI, gender, and prosthetic head size is given in Table 2a. Younger age (OR 0.97, 95%CI 0.94–1.00, p = 0.0453) and smaller head size (OR 0.43, 95%CI 0.19–0.95, p = 0.0367) were associated with an increased risk of dislocation, while BMI and gender did not have an effect on dislocation risk (Table 2b).
Table 1.
Patient Distribution Across Matched Variables Among Cases (Dislocation) and Controls (No Dislocation).
| Matched Variables | No Dislocation |
Dislocation |
p-value | ||
|---|---|---|---|---|---|
| N | % | N | % | ||
| Surgeon | 0.7487 | ||||
| 1 | 9 | 3.5 | 3 | 4.9 | |
| 2 | 29 | 11.7 | 8 | 11.9 | |
| 3 | 5 | 2.0 | 0 | 0 | |
| 4 | 6 | 2.4 | 1 | 1.5 | |
| 5 | 15 | 6.1 | 5 | 7.5 | |
| 6 | 61 | 24.7 | 16 | 23.9 | |
| 7 | 4 | 1.6 | 1 | 1.5 | |
| 8 | 8 | 3.2 | 2 | 3.0 | |
| 9 | 11 | 4.5 | 3 | 4.5 | |
| 10 | 4 | 1.6 | 1 | 1.5 | |
| 11 | 2 | 0.8 | 1 | 1.5 | |
| 12 | 15 | 5.9 | 4 | 6.0 | |
| 13 | 11 | 4.5 | 3 | 4.5 | |
| 14 | 3 | 1.2 | 1 | 1.5 | |
| 15 | 7 | 2.8 | 3 | 4.5 | |
| 16 | 4 | 1.6 | 1 | 1.5 | |
| 17 | 2 | 0.8 | 0 | 0 | |
| 18 | 35 | 14.2 | 10 | 14.9 | |
| 19 | 16 | 6.5 | 4 | 6.0 | |
| Year of surgery | 0.4014 | ||||
| 2002 | 13 | 5.3 | 5 | 7.5 | |
| 2003 | 52 | 21.1 | 15 | 22.4 | |
| 2004 | 23 | 9.3 | 6 | 9.0 | |
| 2005 | 22 | 8.9 | 7 | 10.4 | |
| 2006 | 27 | 10.9 | 9 | 13.4 | |
| 2007 | 13 | 5.3 | 1 | 1.5 | |
| 2008 | 32 | 13.0 | 8 | 11.9 | |
| 2009 | 23 | 9.3 | 5 | 7.5 | |
| 2010 | 26 | 10.5 | 6 | 9.0 | |
| 2011 | 12 | 4.9 | 3 | 4.5 | |
| 2012 | 4 | 1.6 | 1 | 1.5 | |
Table 2a.
Patient Distribution Across Adjusted Covariates Among Cases (Dislocation) and Controls (No Dislocation) With Univariate Analysis of Risk of Dislocation.
| No Dislocation |
Dislocation |
p-value | |||
|---|---|---|---|---|---|
| N | Mean (SD) | N | Mean (SD) | ||
| Age | 247 | 67.0 (11.8) | 67 | 63.6 (12.2) | 0.0968 |
| BMI | 240 | 27.8 (5.4) | 63 | 28.2 (5.2) | 0.2564 |
| Gender | 0.4109 | ||||
| Female | 136 | 55.1 | 42 | 62.7 | |
| Male | 111 | 44.9 | 24 | 35.8 | |
| Head size | 0.0111 | ||||
| <36 | 128 | 51.8 | 48 | 71.6 | |
| > = 36 | 119 | 48.2 | 19 | 28.4 | |
Table 2b.
Multivariate Analysis of Risk of Dislocation Across Adjusted Covariates.
| Logistic Regression Analysis for Dislocation | |||
|---|---|---|---|
| OR | 95%CI | p-value | |
| Age | 0.97 | 0.94–1.00 | 0.0453 |
| BMI | 1.04 | 0.97–1.11 | 0.3007 |
| Gender (F vs. M) | 1.58 | 0.77–3.27 | 0.2164 |
| Head size (> = 36 vs.<36) | 0.43 | 0.19–0.95 | 0.0367 |
The mean acetabular component abduction angle, femoral offset difference, and leg-length difference among cases and controls is given in Table 3a. The results of the logistic regression model adjusted for age, BMI, gender, and head size with acetabular component abduction angle, femoral offset difference, and leg-length difference as risk factors are provided in Table 3b. When femoral offset was at least 3 mm greater than that of the contralateral hip, the risk of dislocation was lower (OR 0.94, 95%CI 0.89–0.99, p = 0.0192). Leg-length difference and acetabular component abduction angle were not associated with dislocation risk.
Table 3a.
Description of Factors on AP Pelvis Radiographs Among Cases (Dislocation) and Controls (No Dislocation) With Univariate Analysis of Risk of Dislocation.
| No Dislocation |
Dislocation |
p-value | |||
|---|---|---|---|---|---|
| N | Mean (SD) | N | Mean (SD) | ||
| Acetabular component abduction angle | 244 | 42.71 (6.67) | 65 | 44.52 (8.33) | 0.0675 |
| Femoral offset difference (operative-nonoperative) | 245 | 3.08 (5.46) | 67 | 1.71 (9.09) | 0.1240 |
| Leg-length difference (operative-nonoperative) | 234 | 3.04 (6.58) | 63 | 0.87 (8.5) | 0.0301 |
Table 3b.
Multivariate Analysis of Risk of Dislocation Across Factors on AP Pelvis Radiographs.
| Logistic Regression Analysis for Dislocation | |||
|---|---|---|---|
| OR | 95%CI | p-value | |
| Acetabular component abduction angle | 1.04 | 0.99–1.09 | 0.1381 |
| Femoral offset difference (operative-nonoperative) | 0.94 | 0.89–0.99 | 0.0192 |
| Leg-length difference (operative-nonoperative) | 0.97 | 0.92–1.01 | 0.1435 |
*Adjusted for age, gender, BMI, and head size.
Acetabular component version was measured for 40 patients (13 cases and 27 controls) for whom cross-table lateral radiographs were available. All acetabular components were anteverted in patients with dislocations. Among patients without dislocations, one acetabular component was retroverted while all others were anteverted. Acetabular component version angle was not associated with dislocation risk (OR = 1.09, 95%CI 0.96–1.22, p = 0.1821).
4. Discussion
Appropriate component position and soft tissue tension must be achieved during THA to minimize the risk of dislocation. The relative contribution of these factors to THA stability, however, is unknown. Our primary study aim was to determine which of acetabular component position, femoral offset restoration, or leg-length equality is most important to THA stability. Secondarily, we assessed if patient characteristics or surgical factors were predictors of THA dislocation.
While this study may provide technical guidance helpful to performing THA, we understand our work has limitations. First, this is a retrospective study limited by the quality of available records. We did not have access to outside hospital records, so all dislocations may not have been captured. Second, we did not measure femoral component version, as a validated method for measurement on AP pelvis radiographs has not been established. Finally, we may not have adequately assessed the effect of significant component malposition on stability, as all study patients were those of high-volume, fellowship-trained surgeons.
To our knowledge, we are the first to report that restoring femoral offset is more important to THA stability than leg-length equality or acetabular component position. Our data do not suggest an association between leg-length and risk of THA dislocation, which is concordant with multiple prior studies.12, 13, 15 Our finding that acetabular component position was not associated with THA dislocation risk is also in agreement with previously published work.14, 16, 17
We found that THA dislocation risk was increased with younger age and smaller prosthetic head size. Others have reported older age to be associated with dislocation, but these studies were case series with heterogeneous populations.18, 19 The observed increased dislocation risk in younger patients may be due to higher activity levels and increased demands at the extremes of hip motion.
Based on our study findings, intraoperative instability during THA should first be addressed by confirming restoration of femoral offset. Leg-length equality and acetabular component position remain important considerations in THA due to patient satisfaction and bearing surface wear/psoas tendon impingement, respectively. Efforts to prevent THA dislocation are particularly important in younger patients, who are at high risk. Future work may assess the role of femoral component version on THA stability and interaction with the factors investigated herein.
Conflict of interest
None.
Funding
There is no funding source.
Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.
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